Hello brains trust. Before I go outside for another spell of troubleshooting in nearly 40 degree heat I thought I'd throw this one out there.
Somewhere in my electrical system there's a leak or bad earth (or other) causing the starter to kick over when I switch the engine off.
It happens like this: when turning the ignition off, as the key passes from ACC to OFF a pulse goes down the IGN (start) wire from the ignition barrel. This is the "exciter" for the starter solenoid relay, so the starter kicks in briefly while the engine is spinning down. I can minimise damage to the starter / ring gear by pausing at ACC until the engine has stopped cranking, then OFF - obviously this is not ideal.
My initial thought was to check the ACC circuit, since it seems related. At the junction where I share the ACC feed from the ignition barrel, I run ACC power to the fuse box for all components that need ACC power. When I disconnect this wire the solenoid no longer fires when turning off - problem solved except I no longer have accessories.
This only used to happen when my heater fan was running as I switched off - managed by turning off the heater first. Since fitting a car stereo and amp (always powered by ACC) it happens all the time. My H4 headlights are also dim (hi beam is fine though), which may or may not be related - bad earth?
I doubt there's a silver bullet here but if anyone has any tips or tricks for troubleshooting this kind of thing, please share. I'm really just fishing around randomly at the moment with my multimeter - hoping there's a more methodical approach I can take.